FIG. 1 is a circuit diagram showing a circuit configuration of a conventional onboard audiovisual system. This apparatus is called as a rear entertainment system in which a driver and rear-seat passengers enjoy listening to audio and watching at video. This kind of system is found also in U.S. Pat. No. 5,661,811.
Referring now to FIG. 1, reference numeral 51 denotes a frequency modulation (FM)/amplitude modulation (AM) tuner, reference numeral 52 a cassette tape player, and reference numeral 53 a compact disc (CD) player or a minidisk (MD) player. Reference numerals 21, 22 denote audio selecting circuits for selecting audio signals reproduced by these audio devices 51, 52, 53. Reference numerals 6a, 6b denote front-seat speakers disposed in positions obliquely before a driver's seat and a front passenger's seat, and a reproduced audio signal of the audio switching circuit 21 is inputted thereto through a volume controller 23 and an amplifier 42. Reference numerals 7a, 7b denote rear-seat speakers, and a reproduced audio signal of the audio selecting circuit 22 is inputted thereto through an amplifier 43 and an audio signal switching circuit 20. In addition, the switched output from the audio signal switching circuit 20 is inputted to headphones 8a, 8b for two persons. Reference numeral 5 denotes a microcomputer which outputs a control signal in response to a command signal to be outputted from a rear-seat controller 9 and a front-seat controller 41 to thereby control the switching operation of the audio selecting circuits 21, 22 and audio switching circuit 20 through lines 25, 26, 27.
The operation of the conventional onboard audio visual system will then be described.
Now, suppose that the audio selecting circuits 21, 22 and the audio switching circuit 20 are selected by driver's operation of the front seat controller 41 as shown in FIG. 1, and that the reproduced signal from the FM/AM tuner 51 is outputted. The driver controls the volume of the front-seat speakers 6a, 6b and the rear-seat speakers 7a, 7b by the volume controller 23, 24. The front-seat speakers 6a, 6b are driven by the output of the amplifier 42 and the rear-seat speakers 7a, 7b by the output of the amplifier 43. The driver can listen to the reproduced audio output from these speakers 6a, 6b, 7a, 7b. 
Here, in case if the rear-seat passengers wish to listen to another audio of an audio device different from that (FM/AM tuner 51) selected by the driver, a command signal is sent to the microcomputer 5 by using the rear-seat controller 9 disposed nearby. Then, in response to the control signal sent from the microcomputer 5 through the signal line 26 shown by a dotted line, the audio selecting circuit 22 selects one of the audio devices 51, 52,53 which the rear-seat passengers want. Further, in response to another control signal sent from the microcomputer 5 through the signal line 27, the audio signal switching circuit 20 switches the output from the rear-seat speakers 7a, 7b to the headphones 8a, 8b in order to output thereto. Therefore, the reproduced audio signal from the selected audio device is amplified by the amplifier 43 and, after the volume is controlled by a controller (not shown), is inputted into the headphones 8a, 8b for outputting a reproduced audio.
Under the state so far mentioned, the audio selecting circuit 21 remains unchanged as selected by the driver and is not switched thereafter. Therefore, the driver will listen only to the audio from the front-seat speakers 6a, 6b. The rear-seat passengers, on the other hand, will listen to the audio selected by themselves with the headphones 8a, 8b. In this manner, the driver and the rear-seat passengers do not suffer from interference in reproduced audio, so that the respective persons can enjoy the reproduced audio output from the respective audio devices.
In the conventional onboard audio visual apparatus thus arranged as above, while the rear-seat passengers enjoy the audio with the headphones selected by themselves, their ears are stopped by the headphones, with the result that the rear-seat passengers fail to hear the foreign sounds such as emergency alarms, or the like received while the motor vehicle is running. Particularly, in case need arose that the driver should have some communication with the rear-seat passengers, the driver faces a difficulty with their communication.
Since the audio is listened to by the headphones, similar problem as the above caused even in an onboard audio visual system in which a video signal output from such as a digital versatile disk (DVD) player is able to listen to and watch on a display disposed in the rear seat.
Further, since cables are laid out inside the motor vehicle from the audio device disposed on the front-seat to the headphones, the freedom in disposing and handling the apparatus is restricted.
Still further, since the rear-seat speakers are not driven while the rear-seat passengers are enjoying the audio with the headphones, the incomplete sound field around the driver's is generated.
This invention has been made to solve the above and other problems and an object thereof is to provide an onboard audio visual system (a system which handles video and audio as well as that which handles only audio are hereinafter generically referred to as “an onboard audio visual system”). This facilitates the communication between the driver and the rear-seat passengers.
Further, another object of the invention is to provide an onboard audio visual system in which the number of cables laid out inside the motor vehicle is reduced, in which the more greater freedom in disposition is accomplished, and in which the use of the system by the rear-seat passengers is made much easier.
Still further, another object of the invention is to provide an onboard audio visual system in which a sound field of good quality is offered not only to the rear-seat but also to the driver.